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Antigens are molecules that can trigger an immune response. They are typically found on the surface of cells, viruses, bacteria, fungi, and other foreign substances. When an antigen enters the body, the immune system recognizes it as foreign and mounts a defense against it. This defense involves the production of antibodies, which are proteins that bind to specific antigens and neutralize them. Antigens can also be part of a person's own cells. In some cases, the immune system can mistakenly identify these self-antigens as foreign and attack them, leading to autoimmune diseases.

The immune system's ability to distinguish between self and non-self antigens is crucial for maintaining health. This recognition is mediated by specialized cells called lymphocytes, which include B cells and T cells. B cells produce antibodies, while T cells can directly kill infected cells or help regulate the immune response. The interaction between antigens and lymphocytes is a complex process that involves a variety of signaling molecules and cellular pathways. Understanding antigens and their role in immunity is essential for developing effective vaccines and treatments for infectious diseases and autoimmune disorders.